(1) Field of the Invention
The present invention relates generally to methods of range determination and more particularly to a method of passive range determination from a moving vessel to an approaching object using only two bearing measurements.
(2) Description of the Prior Art
Determining the range from a moving vessel to an approaching object has long been an important function of navigation and military systems. In terms of seagoing military vessels, a great deal of effort has been devoted to the development of motion analysis and ranging techniques that exclusively utilize passive sensors to generate bearing measurements to an approaching object. It is important to distinguish between two types of passive sensor systems. A first system utilizes spatial separation between hydrophone arrays (often referred to as a wide aperture array). The second system uses a "point-like" array (such as a submarine's spherical array) which hereinafter will be referred to as a conventional array. Since most submarines do not have a wide aperture array, a multitude of methods have been developed for the conventional array. These methods are based on a fundamental requirement of four independent bearing measurements combined with a maneuver of one's own vessel in order to obtain a complete solution for the approaching object (i.e., range, course, speed and bearing). Other techniques, such as Ekelund ranging, require both bearing and bearing rate (i.e., a series of bearing measurements) on each of two legs of one's own course in order to estimate range to an approaching object.
Using a conventional passive bearing sensor, current methods require more than one bearing measurement on each of two vessel legs for range determination. Unfortunately, if the approaching object is on a possible intercept course, e.g. a torpedo, time is of the essence with respect to determining range. Thus, while the aforementioned range determination methods achieve solutions, they do not arrive at a solution to the range question quickly enough for certain time critical environments.